Stabenow, in Grand Rapids on March 19 to talk about the 2014 Farm Bill, said she has “confidence” in the ability of Barra, who became the auto manufacturer's chief executive in January, to make tough decisions during a challenging period for the Detroit company.

Congressional committees and national transportation agency regulators are launching probes to determine, among other things, how long ago the automaker knew of the faulty parts. GM has admitted knowing about the problem for at least a decade.